1. Field of the Invention
The present invention relates to an ink-jet recording ink (hereinafter abbreviated as “ink”) which is suitable for use in an ink-jet recording system, and particularly to an ink which does not cause a problem of curling of a recording medium containing cellulose even when a water-based ink is applied in plenty to the recording medium, and has good responsiveness to high drive frequencies, storage stability upon long-term storage of the ink and resistance to sticking on an ink-jet recording head. The present invention also relates to an ink-jet recording method and an ink-jet recording apparatus using the ink.
2. Related Background Art
An ink-jet recording method is a system wherein minute droplets of ink are ejected to apply to a recording medium such as paper, thereby conducting recording. According to a thermal system wherein electrothermal converters are used as ejection-energy supply means to apply thermal energy to an ink so as to generate bubbles, thereby ejecting droplets of the ink, the formation of a high-density multi-orifice in an ink-jet recording head can be realized with ease. The feature of the thermal system resides in that high-resolution and high-quality images can be recorded at high speed (see, for example, Japanese Patent Publication Nos. S61-59911, S61-59912 and S61-59914).
In recent years, the size of an ink droplet ejected from a single nozzle has come to be made smaller for purpose of providing high-quality ink-jet recorded images of silver halide photograph level. Ink-jet printers, the amount of an ink droplet ejected from which is about 5 pL (picoliters) or smaller, are currently marketed. With respect to a recording speed as well, still higher speeding-up is required. Attending to this requirement, it is of urgent necessity for printers to meet a higher drive frequency and to improve sticking resistance. It is also necessary to always retain an ink in a stable state so as to achieve stable ejection when the ink is left to stand for a long period of time under various environments, namely, to improve the storage stability of the ink.
On the other hand, inks used in ink-jet recording generally comprise water as a principal component and additionally include a water-soluble high-boiling solvent such as glycol for the purpose of, for example, preventing drying and improving resistance to sticking on an ink-jet recording head. When such inks are used to conduct recording on a recording medium containing cellulose, typified by plain paper, lightweight coated paper or the like, there is offered a problem that curling occurs when a great amount of the inks are applied to a region not smaller than that of a certain area in a short period of time. This problem has not been offered in the recording mainly for characters that has heretofore been mainly conducted, since the amount of the inks applied is relatively small. However, this problem is an important problem to be solved upon recording of information with images on a homepage in the Internet, photographic images or the like on plain paper or the like containing cellulose, which requires to apply inks in plenty.
As opposed to this, water-based ink compositions containing various kinds of anti-curling solvents have been proposed (see, for example, Japanese Patent Application Laid-Open Nos. H6-157955 and H11-12520). According to the techniques described in these documents, an effect on curling resistance is recognized to some extent, but further improvements are desired as to simultaneously satisfy responsiveness upon ejection at high drive frequencies, sticking resistance, and curling resistance.
The present inventors have carried out an investigation as to basic properties of an ink-jet recording ink against the problem with the background of a future technical trend as described above. Specifically, the present inventors have carried out a vigorous investigation as to ink-jet recording printers that cause no problem of curling in a recording medium containing cellulose even when a great amount of inks are applied to the recording medium (curling resistance), and can meet high-definition recording of images while retaining good responsiveness to ejection at a high drive frequency, specifically, frequency exceeding 10 kHz (frequency responsiveness), storage stability under a long-term storage of inks, resistance to sticking on an ink-jet recording head (clogging resistance of a nozzle), and the like at high levels. As a result, it has been found that an ink of a composition containing a compound having a particular nature can achieve the above-described object at an extremely high level, thus leading to completion of the present invention. The improvements to be required of an ink will now be outlined.
1. Curling Resistance:
When a great amount of a water-based ink is applied to a recording medium containing cellulose, typified by plain paper, a phenomenon of the so-called curling is easily to be caused, and so the paper may be rounded in a cylindrical form in some cases. The occurrence of the curling is considered to be attributable to such a condition that in a step of drying paper on a paper making stage, water is evaporated in a state that a tension has been applied in a fixed direction, and a hydrogen bond is formed between cellulose molecules. When a water-based ink is applied to the paper of this state, the hydrogen bond between the cellulose molecules is broken by water, and the bonding site is replaced by water. When the water is evaporated, a hydrogen bond is formed again between the cellulose molecules. Since no tension is applied upon the reformation of this hydrogen bond, the paper is shrunk toward the ink-applied side. As a result, it is considered to cause curling.
As described above, the curling phenomenon is hard to be caused in recording mainly for characters because the amount of inks applied to the recording medium is relatively small. However, the curling phenomenon becomes a great problem at the present time the frequency of graphic printing is increasing, and so higher improvement in curling resistance compared with the before is required for ink-jet recording. In particular, it is greatly required to improve the curling resistance upon recording on a recording medium having a recording area of 15 cm2 or more and containing cellulose in an amount of a water-based ink applied within a range of from 0.03 to 30 mg/cm2, further from 0.1 to 20 mg/cm2.
2. Frequency Responsiveness:
When ejection is continuously conducted at a high drive frequency in an On-Demand type ink-jet system, refill of an ink into a nozzle cannot be made in time according to the physical and/or chemical properties of the ink, and so the next ejection may start in some cases before completion of the refill. As a result, ejection failure occurs, or an ejected ink quantity is extremely reduced. This phenomenon is more marked as droplets ejected become smaller.
3. Storage Stability:
It is important to retain an ink in a stable state so as to always achieve stable ejection without causing physical changes such as aggregation and viscosity increase and without being affected by pH change or substances dissolved out of an ink flow path even when the ink is left to stand under low-temperature and high-temperature environments. Incidentally, the storage stability of the ink also greatly affects the following sticking resistance.
4. Sticking Resistance (Clogging Resistance of a Nozzle):
Another problem caused by evaporation of water in an ink that occurs at an orifice includes clogging caused by sticking of a colorant that occurs at the orifice. The following cases are considered as specific examples where sticking resistance is deteriorated.    (1) A case where a printer is left to stand without being used for a certain period of time.    (2) When an ink tank is formed integrally with an ink jet recording head, a case where the ink-jet recording head itself is left to stand in a state removed from a printer.    (3) When an ink tank is separable from an ink-jet recording head, a case where the ink tank is left to stand in a state removed from a printer.
It is accordingly an object of the present invention to provide an ink-jet recording ink that causes no problem of curling in a recording medium containing cellulose, such as plain paper, even when a great amount of inks are applied to the recording medium, and can meet high-definition recording of images while retaining good frequency responsiveness upon ejection at a high drive frequency, storage stability upon long-term storage of the ink, and resistance to sticking on an ink-jet recording head at high levels.
Another object of the present invention is to provide an ink-jet recording method that can stably form a high-quality image.
A further object of the present invention is to provide an ink-jet recording apparatus that can be applied to the ink-jet recording method.
The above objects can be achieved by the present invention described below.